Taste-based repellants
Taste-based repellents generally have a bitter or spicy taste that the deer find offensive.  Most homemade deer repellents fit into this category, especially if you threw in garlic or hot peppers.

Although deer sometimes steer clear of these products, the nibblers have to take a nice big bite of your favorite tulips in order to learn the plants are no longer tasty.  And hungry deer --- like most deer in overpopulated areas in the winter --- will eat nasty plants despite the repellent because they're starving.

Trying to fill your garden with plants deer naturally find offensive, like daffodils, runs into a similar problem.

On to scent-based repellents...